1. Field of the Invention
This invention relates to a color-image processing apparatus. More specifically, the invention relates to color-data correction processing capable of reproducing a stable variable-density image.
2. Description of the Related Art
An image processing unit of a conventional color-image reproducing apparatus has a configuration shown in FIG. 1.
In FIG. 1, a color processing unit 10 receives R (red), G (green) and B (blue) image signals, each comprising 8 bits, i.e., 24 bits in total, and converts the received signals into an 8-bit VDO (video) signal, comprising a Y (yellow) signal, an M (magenta) signal, a C (cyan) signal or a BK (black) signal, in accordance with a color assigning signal.
Each VDO signal, comprising a Y signal, an M signal, a C signal or a BK signal, is subjected to .gamma. correction by a -correction unit 11 so that the correlation between density data and the density of actually printed data is close to linear, and is converted into an 8-bit signal.
Since each of Y, M, C and BK colors has its peculiar density curve (the correlation between density-level data and the density of actually printed data), the .gamma.-correction unit 11 includes a table, called a .gamma. table, for each color, and correction is performed by multiplying an input signal of each color by a .gamma. coefficient.
In the configuration shown in FIG. 1, a relatively stable image is obtained in the case of a color-image reproducing apparatus only having a low-resolution printing capability. On the other hand, in the case of an image reproducing apparatus having a high resolution of about 600 dpi (dots per inch), the size of one pixel (picture element) is considerably reduced. Accordingly, when the density level is lower than a certain level, the size of a printed dot becomes very small, so that the dot is not printed in a stable manner. Hence, low-density portions in an image are not stably printed, so that the difference between high-density portions and low-density portions is unstable. Moreover, low-density portions under a certain density level are not printed at all.
The stability differs for each of Y, M, C and BK colors. For example, while Y images are relatively stably printed from high-density regions up to low-density regions, BK, M and C images have inferior stability.
Particularly, intermediate-density regions are unstably printed in the case of M images, and instability in low-density regions is pronounced in the case of C images.
Since visual impression differs for respective colors, the above-described instability in respective colors causes the problem that a printed image has a color tone different from that of an original image. Color-image reproducing apparatuses have the above-described problems.